Various techniques of passively mode locking lasers have been developed which use two coupled cavities, one with gain and one with a nonlinear optical element or mirror which returns to the laser cavity an optical pulse shorter in duration than that which was incident on it. Stankov, "A Mirror with an Intensity-Dependent Reflection Coefficient," Applied Physics B, Vol. 45, pp. 191-195 (1988), describes the use of one type of nonlinear mirror with power (intensity) dependent reflection to mode-lock a Nd:YAG laser and thereby obtain short light pulses. In the Stankov device, an intense light beam at frequency .omega. generates a second harmonic beam in a nonlinear crystal. The total second harmonic at 2.omega. and part of the fundamental beam are reflected by a dichroic mirror back through a phase-adjusting glass plate to provide the necessary phase relation between the two reflected light waves, and then back through the nonlinear crystal. In the second pass through the crystal, partial reconversion of the second harmonic into the fundamental wavelength takes place. The degree of conversion and reconversion is dependent on the intensity of the incident beam.
Another example is the additive pulse or coupled cavity laser, in which the main laser cavity is coupled to an external cavity containing the nonlinear element. Pulses which are altered in phase or amplitude in the external cavity combine at the output coupler with the pulses in the main cavity so that the central, high intensity portions of the pulses are preferentially returned to the main cavity. See Mark, et al., "Femtosecond pulse generation in a laser with a nonlinear external resonator," Optics Letters, Vol. 14, pp. 48-50 (1989); Kean, et al., "Enhanced modelocking of color-center lasers," Optics Letters, Vol. 14, pp. 39-41 (1989); and Barr et al., "Coupled cavity modelocking of a Nd:YAG laser using second harmonic generation," Optical Society of America Conference on Lasers and Electro-Optics. 1989 Technical Digest Series, Vol. 11, p. 442 (1989).
U.S. Pat. No. 4,853,933 to Blow et al. discloses another coupled cavity mode locked laser in which the external cavity comprises a nonlinear element into which the beam reflected from a splitter is injected, and a mirror downstream of the nonlinear element. The nonlinear element is either a dispersive or nondispersive medium in which the nonlinear loss or transmission effects predominate over any dispersive effects and nonlinear refractive index effects.
Such lasers have produced nearly transform-limited pulses, and show great promise in being able to utilize the gain bandwidth of solid state lasers to produce the shortest pulses possible from this type of laser. However, such lasers typically require active feedback circuits (length stabilization) to maintain the proper phase relationship between the two cavities. In addition, active mode locking may also be required to produce sufficiently high peak powers to initiate the nonlinear process. See Ippen et al., "Additive pulse modelocking," Journal of Optical Society of America B, Vol. 6, pp. 1736-1745 (1989).
It was also known that optical soliton switching could be achieved in an all-fiber nonlinear loop mirror or Sagnac (antiresonant ring) interferometer. See Doran, et al., "Nonlinear-optical loop mirror," Optics Letters, Vol. 13, No. 1, pp. 56-58 (January 1988); and Blow, et al., "Experimental demonstration of optical soliton switching in an all-fiber nonlinear Sagnac interferometer," Optics Letters, Vol. 14, No. 14, pp. 754-756 (July 1989). Similarly, Siegman, "An antiresonant ring interferometer for coupled laser cavities, laser output coupling, modelocking, and cavity dumping," Journal of Quantum Electronics, pp. 247-250 (February 1973) discloses various linear antiresonant ring mirror embodiments for use in lasers which include the use of frequency doubling crystals to obtain a laser output at a doubled frequency. In one embodiment, a phase shifter is also provided to shift the relative phases between the second harmonic and the fundamental frequency beams within the ring so that the counterpropagating beams add or interfere such that the second harmonic beam is rejected from the ring and all of the fundamental beam is returned to the laser cavity. Also, Otsuka, "Nonlinear antiresonant ring interferometer," Optics Letters, Vol. 8, pp. 471-473 (1983) discloses a transmissive antiresonant ring with a Kerr effect medium and an unbalanced beam splitter for use in switching and logic operations.